This invention is an improvement on a gear generating machine for cutting spur and helical gears and other toothed elements, and more particularly wherein the machine comprises an endless chain of cutting tools.
Such a machine is disclosed in U.S. Pat. No. 1,469,602. A recent invention comprising improvements making a machine of the endless chain type capable of cutting gear teeth by intermittent indexing methods as well as by the continuous indexing method, is disclosed in co-pending application Ser. No. 929,600, filed July 31, 1978.
In machines of a more conventional type which employ rotating hobbing cutters, particularly of the cylindrical type, as contrasted with machines of the type disclosed herein employing the endless chain cutter, it is well known to add life to cutting tools by adjusting the hob relative to successive workpieces by shifting the hob axially a controlled amount every so many workpieces cut. It is also known in such hobbing machines to provide a mechanism for producing the periodic shift automatically. Such mechanisms are disclosed, for example in U.S. Pat Nos. 2,481,974, 2,697,382 and 4,082,031. It is also known to effect the axial shift of the hob as a continuous motion during cutting, and an example of such an apparatus will be found in U.S. Pat. No. 2,537,967. The shifting is done so as to better distribute the wear over the full face of the hob.
In the chain-generating machine of the present invention, the tools of the chain travel in a flat plane in the cutting zone, and the endless chain of cutters provides, in effect, a cutter of infinite diameter, having many times more cutting edges than a conventional hob. Generally, a stack of several workpieces is cut in a single operation, and all the workpieces in the stack are engaged simultaneously by the chain cutter, there being no feed motion axially of the workpiece.
The machine has great inherent advantages over a hobbing machine, and to obtain the maximum advantages available in productivity, quality of finish, and tool life, it is necessary to make a shift of the chain cutter relative to the work. That is, in order to realize the ultimate obtainable gains, it is necessary to manage the distribution of the wear on all the available cutting edges of the chain cutter, as it is necessary to do in a conventional hobbing machine in order to obtain the best economy available with that type machine. This part of the problem is much the same, in that a periodic relative shift of cutter and work in the direction corresponding to the hob axial direction in a conventional hobber, is needed.
As noted above, in hobbing machines, the shifting to properly distribute wear is done by moving the hob along its axis. Various mechanisms are employed to accomplish the shift automatically. For example, in the mechanism disclosed in U.S. Pat. No. 2,481,974, at the end of the hobbing cycle a reversible motor rotates a worm wheel-rachet combination which locks and unlocks and actuates a screw and nut thrust means to advance the hob arbor and the hob a predetermined amount along the hob axis.
In another hob shifting arrangement, disclosed in U.S. Pat. No. 2,697,382, automatic hydraulic controls reversibly actuate a piston which is integral with a wide faced helical gear. The helical gear engages a rack fixed to a hob slide operable in the axial direction of the hob. As the piston is actuated in one direction, the piston-gear serves to advance the rack and the hob through a small predetermined distance.
In U.S. Pat. No. 4,082,031 a hob slide, for moving a hob in its axial direction, is moved incrementally by a screw and nut means rotated, through gearing, by a stepping motor.
Although the chain cutter employed in the continuous indexing operation has a basic similarity to a hob, a problem exists which does not present itself in the conventional hobbing machines. Due to the chain drive system, the size and weight of the chain cutter and its supporting structure, and the heavy cutting loads, considerations of accuracy and rigidity make it impractical to provide for a shift movement of the chain.
However, the invention as described herein, obtains the necessary relative movement by means of a mechanism which provides for intermittent movement of the work spindle laterally in a plane parallel to the plane in which the tools travel while cutting. Since there is a component of this shifting movement which is at right angles to the direction of the chain travel, there is thus provided a relative cutter and work displacement which is comparable to the axial shift of the hob in the conventional hobber.
In this way, increased tool life is obtained without shifting the chain support and, therefore, without interfering with the rigidity of the chain drive system and of the support structure, which are vital to machine accuracy.
The machine and the apparatus of the invention will be further described with reference to the accompanying drawings.